How to draw rectangle in a plane?
This is a great question, and had to think about it, but I think I have a simple solution:

Go to Draw Mode
With the box tool draw a rectangle at any location and orientation you want in the 3D Viewport.
Hit Enter to confirm the box location, orientation, size
Optional: change your view so you can understand this technique is not dependent on world view
Enter Edit Mode
With the Selection Tool active set the Selection Mode to Select Only Points or Select all Stroke Point



Select the whole rectangle you just created

Now switch the Cursor tool

With the Cursor tool selected go to the tools Settings and and set the Transformation Orientaion to Local




From the 3D Viewport Menu chose Grease Pencil &gt; Snap &gt; Cursor To Selected (you can also use the shortcut key: Shift + S  )



Go back to Draw Mode
Draw with any tool on the rectangle and then move your view to see that the strokes are in the same drawing plane.

You can see the drawing plane by turning on the 3D Canvas in the Viewport Overlay (In the Edit Grease Pencil section, put a checkmark next to Canvas to turn it on)